Apparatus and method for programmable line interface impedance matching

ABSTRACT

A line interface capable of connecting to a variety of transport mediums, each having a different impedance. The line interface comprises a programmable resistor. The programmable resistor along with an external resistor provide a range of resistor values which are used to substantially match the impedance requirements of the various transport mediums.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of network circuitry. Moreparticularly, the invention relates to a more versatile line interfacefor connecting a receiver to various transport mediums (e.g., datalines) hosting transport protocols that have different impedancerequirements.

2. Description of Related Art

Various transport protocols are used to transport data across transportmediums. Transport protocols are defined by various industryspecifications. For example, the T1 protocol (also called DS1) isspecified by the American National Standards Institute (latest revisionT1.403.00, 403.01, 403.02-1999). The E1 protocol (or E-1) is a Europeandigital transmission format devised by the InternationalTelecommunication Union (ITU-T) and is compliant with G703 (latestrevision October, 1998). Similarly, the J1 protocol, used in Japan, isspecified by the Telecommunication Technology Committee (latest revisionJT-G703, April, 1989).

When a connection is made to a transport medium hosting a particulartransport protocol, the line interface, or connector, must have asubstantially similar impedance to that specified by the transportprotocol, otherwise electrical reflection will occur causinginterference, as is well known.

FIG. 1 shows a prior art line interface for connecting a receiver to atransport medium (line 110). The line 110 is connected to a tip input140 and a ring input 142 of the receiver portion of transceiver 150 viaa transformer 120. A resistor 130 is used to provide an impedance formatching the impedance of line 110. For example, a T1 line according tothe ANSI T1 specification has a nominal terminating impedance at theinterface of 100 ohms. The T1 specification also specifies a return losswith respect to 100 ohms over the frequency band from 100 kHz to 1 Mhzof at least 26 dB.

The return loss is determined by the equation:

20 log10 [(Z_(T)+Z_(L))/Abs(Z_(T)−Z_(L))], where Z_(T) is the impedanceof the line interface, Z_(L) is the impedance of the line, and Abs isthe absolute value function. It is clear that the closer Z_(T) is toZ_(L), the higher the return loss. If Z_(T) is not close enough toZ_(L), the return loss requirement of the T1 specification will not bemet. The term “substantially match” used herein denotes meeting thespecification of the transport protocol.

In this prior art line interface, if the transceiver is subsequentlyconnected to a transport medium that has a different impedancerequirement, then the external resistor is physically replaced with acorresponding resistor. For example, a J1 line requires a nominaltermination impedance of 110 ohms; an E1 line (coax line) requires anominal termination impedance of 75 ohms; and an E1 line (twisted pairline) requires a nominal termination impedance of 120 ohms. In the priorart line interface, the external resistor would be switched to match thenominal termination impedance of the new transport medium and transportprotocol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art line interface for connecting a receiver to atransport medium.

FIG. 2 shows an exemplary line interface for a receiver having aprogrammable resistor to receive data across transport mediums havingdifferent impedance requirements.

FIG. 3 shows a block diagram of one embodiment of an exemplaryadjustable resistor circuit R_(Y) in an IC receiver.

FIG. 4 shows a flowchart showing an example of setting the effectivetermination resistance RT to a predetermined value.

FIG. 5 shows one embodiment of resistor circuit block 310 that allowstrimming of the resistance.

FIG. 6 shows a flowchart of the process of trimming one of the resistorcircuit blocks 310 and 320 of the adjustable resistance R_(Y).

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout some of these specific details. In other instances, well-knownstructures and devices are shown in block diagram form to avoidobscuring the underlying principles of the present invention.

An Exemplary Line Interface

FIG. 2 shows an exemplary line interface for a receiver having aprogrammable resistor to receive data across transport mediums havingdifferent impedance requirements. For example, a transport medium suchas line 210 may carry any of several different transport protocols suchas those previously mentioned. In one case, line 210 may carry atransport protocol having an impedance requirement of 100 ohms. Inanother case, or at a different point in time, the line 210 may carry atransport protocol having an impedance requirement of 110 ohms.Alternatively, the line 210 may have an impedance requirement of 120ohms.

A 1:1 transformer 220 and an external resistor 230 couple line 210 to areceiver portion of integrated circuit (IC) transceiver 250 at receivertip and ring inputs 240 and 242. The IC transceiver has an adjustableresistance R_(Y) 260 that is electrically programmable. The combinedeffective resistance of both the external resistor 230 in parallel withthe adjustable resistance R_(Y) 260 is used to match the impedance ofthe line 210. Because the IC transceiver 250 has an adjustableresistance R_(Y), the line interface 200 is able to match the impedancesof line 210 for various transport protocols and transport mediumswithout the need to physically change the external resistor 230.

The use of an adjustable resistance R_(Y) 260 in the IC transceiverfacilitates the process of switching from one transport medium toanother since modification of the resistance can be done in software.Additionally, by keeping adjustable resistance R_(Y) 260 relatively lowcompared to R_(X), the majority of power dissipation will be in theexternal resistor R_(X) and not in the IC.

FIG. 3 shows a block diagram of one embodiment of an exemplaryadjustable resistor circuit R_(Y) 260 in an IC receiver 250. Oneresistor circuit block 310 is enabled to provide a first IC resistance.Alternatively, the other resistor circuit block 320 is enabled toprovide a second IC resistance. A register 330 is used to select whethereither resistor circuit block 310 or 320 is enabled or disabled. This isdone via transmission gates or other well-known means.

In this embodiment, an effective termination resistance R_(T) comprisesthe internal IC resistance in parallel with the external resistanceR_(X). Thus, the effective termination resistance of the parallelresistors is

-   -   R_(T)=R_(X)R_(Y)/(R_(X)+R_(Y)), where R_(X) is the external        resistance, and R_(Y) is the internal IC resistance.

If both resistor circuit blocks 310 and 320 are disabled, then R_(Y) isan open circuit and the effective termination resistance R_(T) is merelythe external resistor R_(X).

In one embodiment, the external resistor R_(X) has a value of 121 ohms+/−1%, and resistor circuit blocks 310 and 320 in parallel with R_(X)provide an effective termination resistance R_(T) of 100 ohms +/−5% and110 ohms +/−5%, respectively. If both resistor blocks 310 and 320 aredisabled, an effective termination resistance R_(T) of 120 ohms +/−5% isprovided. Of course, additional resistor blocks can be added and/or theresistor blocks could provide other effective termination resistancevalues such as 75 ohms (e.g., for an E1 coaxial cable). However, lowerresistances 110 dissipate more power in the receiver. One alternativefor the line termination to accommodate 75 ohms +/−5% is by inserting anexternal balun (transformer) between the line 210 and the transformer230. The programmable resistor R, and an external balun can be used incombination for providing a 75 ohms +/−5% line termination.

FIG. 4 shows a flowchart showing an example of setting the effectivetermination resistance R_(T) to a predetermined value. Typically thisprocedure is done upon initialization when a new line 210 is attached tothe transceiver.

The flowchart starts at box 400 and proceeds to box 410 at which a writeto register 330 is performed. The bit value written to register 330determines whether to enable either resistor circuit 310 or 320 orneither circuit block based on the desired R_(T) to match the new line210. Based on the value written to register 330, the adjustable resistorR_(Y)'s value is changed as shown in box 420.

FIG. 5 shows one embodiment of resistor circuit block 310 that allowstrimming of the resistance. Trimming is a way of fine tuning theresistance value of the resistor circuit because the resistance providedby resistors on an IC may vary by up to around 20% due to variations inprocess, temperature, and voltage. In one embodiment, seven pairs ofresistors are coupled in parallel with transmission gates M15, M10, M5,0, P5, P10, and P15 between the resistor pairs as shown in FIG. 5. Atrim register (not shown) is used to enable or disable particulartransmission gates. For example, various combinations of 3-bit valueswritten to the trim register can result in each of the resistorconfigurations shown in Table 1 (below). In this embodiment, theresistors are set up so that a nominal resistance R_(YNOM) is providedwhen transmission gates 0, P5, P10 and P15 are enabled, but transmissiongates M5, M10 and M15 are disabled. In this embodiment, if the nominalresistance R_(YNOM) differs from the resistance value desired, then aresistor is either enabled to lower R_(YNOM) or disabled to raiseR_(YNOM).

For example, in this embodiment, the value of R_(M5) is selected suchthat when it is enabled (by enabling gate M5), R_(YNOM) is reduced by5%. Similarly, R_(M10) is selected such that when it is enabled,R_(YNOM) is reduced by 10%; and R_(M15) reduces R_(YNOM) by 15% whenenabled. Additionally, R_(P5) is selected such that when it is disabled,R_(YNOM) increases by 5%; R_(P10) when disabled increases R_(YNOM) by10%; and R_(P15) when disabled increases R_(YNOM) by 15%. Table 1 showsthe various configurations of transmission gates and the correspondingresistance of the adjustable resistor circuit as was just described. An‘X’ in the table indicates the transmission gate is enabled. It shouldbe noted that there is no IC resistor configuration that uses a singlepath of resistance between RTIP 240 and RRING 242. This helps reduceelectron migration which improves the lifetime of the IC resistors.

TABLE 1 R_(M15) R_(M10) R_(M5) R₀ R_(P5) R_(P10) R_(P15) Resistance X XX X R_(YNOM) X X X 1.05 R_(YNOM) X X X 1.10 R_(YNOM) X X X 1.15 R_(YNOM)X X X X X .95 R_(YNOM) X X X X X .90 R_(YNOM) X X X X X .85 R_(YNOM)

Because the total current flowing through parallel resistors is the sameas the current flowing through an equivalent resistor, the followingequationsI=V/R _(EQ) =V/R 1+V/R 2+V/R 3+ . . .and R _(EQ)=1/(1/R 1+1/R 2+1/R 3+ . . . )allow us to convert the various configurations of enabled resistorsshown in Table 1 into the following equations:1/((½R ₀)+(½R _(P5))+(½R _(P10))+(½R _(P15)))=R _(YNOM)  (EQ. 2)1/((½_(R0))+(½R _(P10))+(½R _(P15)))=1.05R _(YNOM)  (EQ. 3)1/((½R ₀)+(½R _(P5))+(½R _(P15)))=1.10R _(YNOM)  (EQ. 4)1/((½R ₀)+(½R _(P5))+(½R _(P10)))=1.15R _(YNOM)  (EQ. 5)1/((½R _(M5))+(½R ₀)+(½R _(P5))+(½R _(P10))+(½R _(P15)))=0.95R_(YNOM)  (EQ. 6)1/((½R _(M10))+(½R ₀)+(½R _(P5))+(½R _(P10))+(½R _(P15)))=0.90R_(YNOM)  (EQ. 7)1/((½R _(M15))+(½R ₀)+(½R _(P5))+(½R _(P10))+(½R _(P15)))=0.85R_(YNOM)  (EQ. 8)

If R₀ is chosen to be the smallest resistor in the resistor circuit andis used as a unit resistor, then R_(M5)=k_(M5)*R₀, R_(M10)=k_(M10)*R₀,R_(M15)=k_(M15)*R₀, R_(P5)=k_(P5)*R₀, R_(P10)=k_(P10)*R₀, andR_(P15)=k_(P15)*R₀, where k_(M5), k_(M10), k_(M15), k_(P5), k_(P10), andk_(P15) are unit resistor coefficients. Solving for equations 2-8 forthe unit resistor coefficients yields the following approximations(rounded to nearest half integer): R_(M5)=14*R₀, R_(M10)=6.5*R₀,R_(M15)=4*R₀, R_(P5)=15*R₀, R_(P10)=8*R₀, R_(P15)=5.5*R₀.

As an example, for an effective termination resistance of 100 ohms, andexternal resistor R_(X) of 121 ohms:R _(YNOM)=(R _(X) *R _(T))/(R _(X) −R _(T))=(121*100)/(121−100)=576ohms, andR ₀=396 ohms.

For an effective termination resistance of 110 ohms, and externalresistor R_(X) of 121 ohms:R _(YNOM)=(R _(X) *R _(T))/(R _(X) −R _(T))=(121*110)/(121−110)=1210ohms, andR ₀=831 ohms.

Note that for the actual implementation of the programmable integratedcircuit resistor, the unit resistor values are adjusted for the finiteon-resistance of the transmission gates and for the rounding of the unitresistor coefficients. The resistor values may also be adjusted forcontact and metal resistances.

FIG. 6 shows a flowchart showing the process of trimming one of thecircuit blocks 310 or 320. In one embodiment, both resistor circuitblocks 310 and 320 are trimmed identically since the resistancevariation due to process is consistent across the IC. Thus, the sameresistors would be enabled and disabled in each resistor circuit block.

The flowchart starts at block 600 and proceeds to block 610 at which theIC resistance R_(Y) is determined. This may be done using an IC testeror an ohm-meter. Once the value of the resistor circuit is determined,it may be modified in block 620 by enabling or disabling the appropriateresistor to decrease or increase the resistance. In one embodiment, onlyone resistor is enabled or disabled, and each of the resistors adds anincrement or decrement of a predetermined percentage of the resistance.For example, the resistance can be increased or decreased by 5%, 10%, or15%.

After the resistance has been trimmed to the desired value, furthertrimming of the resistance value R_(Y) may optionally be disabled (block630). This may be done by a variety of different methods includingblowing a fuse on the IC. Subsequently, a resistor circuit block couldbe enabled or disabled but the resistance 110 value of the resistorcircuit block could no longer be trimmed.

Thus, an apparatus and method for a programmable line interface forimpedance matching is described. Throughout the foregoing description,for the purposes of explanation, numerous specific details were setforth in order to provide a thorough understanding of the invention. Itwill be apparent, however, to one skilled in the art that the inventionmay be practiced without some of these specific details. For example,the programmable resistance could be accomplished by resistors in seriesas well as in parallel; and the IC may have multiple independentreceivers and/or transceivers, each capable of coupling to lineinterfaces with different effective termination resistances.Accordingly, the scope and spirit of the invention should be judged interms of the claims that follow.

1. In a line interface having a programmable resistor, a method ofmatching an impedance of a transport medium comprising: determining theresistance of the IC corresponding to a first configuration of parallelresistors, wherein a portion of the parallel resistors are enabled, atleast one or more of the parallel resistors being disposed within theIC; modifying the resistance of the IC by creating a secondconfiguration of parallel resistors, wherein a different portion of theparallel resistors are enabled; and wherein a desired impedance isprovided by a combination of the resistance of the IC and an externalterminating impedance external to the IC.
 2. The method of claim 1wherein the modifying the resistance is performed by writing to aregister on the IC.
 3. The method of claim 1 further comprising:permanently disabling a subsequent modification of the secondconfiguration of parallel resistors.
 4. The method of claim 3 furthercomprising: controlling the entire second configuration of parallelresistors to be enabled and disabled.
 5. The method of claim 3, whereinpermanently disabling of a subsequent modification is achieved byblowing a fuse on the IC.
 6. The method of claim 1, wherein modifyingthe resistance of the IC is performed by enabling a resistor of theparallel resistors to reduce the resistance of the IC by a professionalpercentage.
 7. The method of claim 1, wherein modifying the resistanceof the IC is performed by disabling a resistor of the parallel resistorsto increase the resistance of the IC by a predetermined percentage. 8.In a line interface having a programmable resistor, a method of matchingan impedance of a transport medium comprising: writing to a registerthat controls the programmable resistor, wherein the programmableresistor is disposed within an integrated circuit to couple to thetransport medium; and changing the programmable resistor to provide aneffective impedance substantially matching the impedance of thetransport medium responsive to writing to the register, wherein theeffective impedance is provided by a combination of the programmableresistor and a terminating impedance external to the integrated circuit.9. The method of claim 8 wherein changing the programmable resistor isaccomplished by disabling the programmable resistor.
 10. The method ofclaim 8 further comprising: coupling the line interface to the transportmedium.
 11. The method of claim 10, wherein the transport mediumsupports a T1, J1, or E1 transport protocol.
 12. The method of claim 8wherein the programmable resistor is changed to provide the effectiveimpedance of 75 ohms, 110 ohms, or 120 ohms.